Poly(lactic acid), or poly(lactide), commonly referred to as PLA, is a commercially important biodegradable polyester that has many potential medical, agricultural, and packaging applications because of its biocompatibility and biodegradability. Concern about the environmental impact and increasing cost of petroleum based polymers has renewed interest in polymers derived from natural products, such as PLA.

PLA is produced by the ring opening polymerization (ROP) of the six-membered cyclic ester lactide (Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147-6176; Gupta, B.; Revagade, N.; Hilborn, J. Prog. Poly. Sci. 2007, 32, 455-482; Oh, J. K. Soft Matter 2011, 7, 5096-5108). Lactic acid (LA) is produced in chiral and racemic forms by fermentation of corn and other agricultural products. Lactides are the cyclic diesters of lactic acid and are prepared by the dehydration of lactic acid. When lactide is prepared from racemic lactic acid, the three isomers that result are R-lactide (D-lactide), S-lactide (L-lactide) and meso-lactide; rac-lactide is a 50:50 mixture of R-lactide and S-lactide.

The stereochemistry of PLAs determines, at least in part, their mechanical, physical and thermal properties, as well as their rates of degradation. The bulk properties of PLAs, especially their melting points, are intrinsically linked to the polymer microstructure. Poly(R-lactic acid) and poly(S-lactic acid) are both crystalline polymers with melting points of about 180° C., while atactic PLA produced from the polymerization of RS-lactide is an amorphous polymer with no melting point. The ability to control the polymer tacticity can have an enormous impact on the properties and applications of the final polymer (Dijkstra, P. J.; Du, H. Z.; Feijen, J. Polym. Chem. 2011, 2, 520-527; Buffet, J. C.; Okuda, J. Polym. Chem. 2011, 2, 2758-2763; Thomas, C. M. Chem. Soc. Rev. 2010, 39, 165-173; Stanford, M. J.; Dove, A. P. Chem. Soc. Rev. 2010, 39, 486-494).
Isotactic PLA derived solely from L-lactide (Pm=0.8, where Pm is the probability of finding a pair of adjacent structural units in a polymer that have the same stereochemistry) has a melting point of 178° C., while all heterotactic polymers generated to date through chain end control are amorphous (Buffet, J. C.; Okuda, J. Polym. Chem. 2011, 2, 2758-2763; Fukushima, K; Kimura, Y. Polym. Int. 2006, 55, 626-642). Stereoblock polymers, generated from rac-LA using selective chiral aluminum salen complexes, can have melting points of well over 200° C., displaying the power of stereoselective ROP catalysts (Fukushima, K.; Kimura, Y. Polym. Int. 2006, 55, 626-642).
Chiral catalysts can be used to selectively polymerize one stereoisomer in a racemic mixture of lactides to produce isotactically enriched PLA. For example, metal-salen complexes have been used in asymmetric catalysis, including stereoselective polymerization of rac-lactide (Canali, L.; Sherrington, D. C. Chem. Soc. Rev. 1999, 28, 85; Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147).
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.